The invention concerns methods and apparatus for the processing of solid materials, including hard tissues, metals, ceramics, crystals, glass, certain plastics, etc. and uses thereof in dentistry, surgery, orthopedics and other material processing applications.
Laser radiation is widely used for the processing of hard materials: drilling, cutting, modification of properties and other operations. The mechanism for destruction of hard materials under the influence of laser radiation involves the absorption of laser energy, which results in heating, melting and evaporation of the materials. Other mechanisms involve absorption of radiation by strongly absorptive materials (chromophores), their heating and the breaking of the material because of pressure around the absorptive materials. The process of laser destruction of materials under the influence of short pulses (generally pulses shorter than the thermal relaxation time of the target) is sometimes called laser ablation. In order to reach the maximum efficiency of material removal, the wavelength of the laser radiation is selected to be within the range of maximum absorption for the absorptive material. Depending on the properties of the material, the optimum parameters of laser radiation are selected. These parameters include the wavelength, the pulse duration, the diameter of laser beam spot, and the energy or power. Laser destruction of hard materials has a lot of advantages; however, in many cases it is slower than drilling or other mechanical methods of processing.
Russian certificate of invention USSR N 1593669, published Sep. 23, 1990, discusses the removal of hard tooth tissues by radiation with 2.94 xcexcm wavelength (Er:YAG laser), with pulse duration of 100-500 xcexcs and with energy of 0.5-1 J. U.S. Pat. No. 5,257,935 issued Oct. 2, 1993 proposes a laser with a wavelength within the range 1.5-3.5 xcexcm, in particular 2.94 xcexcm, for the same objective. The radiation in this device is delivered from the laser to the processing zone using an optical fiber connected to a tip in contact with a tooth surface. The disadvantage of this method and apparatus is that the speed of material removal is slower than for high-speed drills. Its use therefore results in an increase in procedure duration. However, the laser procedure is in most cases painless and does not require anesthesia. The laser processing is also less traumatic.
In the apparatus and method disclosed in the U.S. Pat. No. 5,409,376, issued Apr. 25, 1995, mechanical drilling is combined with laser drilling in order to increase the speed of processing. However, this increases the price of both the treatment process and the drilling apparatus. Further, when used for the processing of dental tissues, it results in the loss of the main advantages of laser processingxe2x80x94absence of pain and low danger of trauma.
A major disadvantage of the techniques discussed above is insufficient utilization of the laser energy. This is due to the fact that a significant part of the laser pulse energy absorbed by the processed material is transformed to mechanical energy of particles leaving the zone of processing, this energy being uselessly spent in heating the environment. Similar issues can arise when a laser is used to ablate solid materials other than dental tissue.
In accordance with the above, this invention, in accordance with a first aspect thereof, provides a method of processing a solid material which includes exposing the material to pulsed radiation with an energy above an ablation threshold for the material; and returning or otherwise directing particles of ablated material to a region of processing of the material to further influence material processing. Some of the particles of ablated material will be deposited on a surface adjacent the region of processing, the method including returning these deposited particle to the region of processing in response to the next radiation pulse to further process the material. While the region of processing is the source of the particles of ablated material for a preferred embodiment, other sources of particles may also exist, either in addition to or instead of the preferred source, which particles can be delivered to the region of processing for the further processing of the material. Potential sources for such added material include a tip through which radiation is delivered, reflectors surrounding the tip and/or an additional piece of material positioned between the radiation source and the region of processing which may be ablated by radiation passing therethrough to produce accelerated particles. For a preferred embodiment, the material being processed is a dental material, for example dental enamel, dentin, bone, stain, filling material, cementum and the like. For such embodiments, the pulsed radiation is preferably from a laser with a wavelength within one of the bands 1.9-2.1 xcexcm, 2.65-3.5 xcexcm, 5.6-7.5 xcexcm, and 8.5-11 xcexcm; a duration of 0.0001-10000 xcexcs (preferably 1-500 xcexcs); and an energy density of 0.5-500 J/cm2. The method may also include cooling the region of processing of the material and/or removing particles from an area between a source of the pulsed radiation and the region of processing, these steps preferably being performed between pulses of the radiation for some embodiments. For another embodiment, air is first applied to the region of processing to clean at least the area. A light water spray or mist is then applied to both cool the area and to be ablated, the laser or other radiation source being fired during the applications of the mist. After the firing of the radiation source, the misting or a stronger water spray may be applied to cool the region of processing. While the three steps indicated above are preferably used together, for some embodiments, one or more of these steps may be individually performed.
The invention also includes a device for processing a solid material which includes a source of pulsed radiation and a system for delivering radiation from the source through a tip to a region of processing, the tip including an end for delivering radiation to at least one particle source, the radiation accelerating particles from the particle source, which particles are accelerated and/or reflected to a region of processing on the surface of the solid material to influence the processing thereof. For preferred embodiments, the particle source is the region of processing on the surface of the solid material, the radiation ablating the surface to create particles of ablation accelerated away from the surface, at least some of these particles being reflected back to the region of processing by at least one of the tip and a reflector surrounding the tip to further process the surface. The radiation and the reflected particles may impinge on substantially the same point in a region of processing or they may impinge on different points in this region to increase the area being processed.
At the end of at least some radiation pulses, some particles of ablation may adhere to the tip or other surfaces adjacent the area of processing, and these adhered particles may serve as an additional particle source for a subsequent radiation pulse, the adhered particles being ablated by such radiation pulse so as to be accelerated toward the region of processing. For some embodiments, the tip has an end facet shape to function as a reflector for the particles. At least a portion of the tip may also be ablated by the radiation, the ablated portion of the tip being a source of particles for delivery to the region of processing. A unit may also be positioned between the tip and the region of processing which unit is ablated by radiation applied thereto to produce particles of ablation directed to the region of processing. A mechanism may be provided for advancing the portion of the unit between the tip and the region of processing as the unit is ablated. For preferred embodiments, the source of pulsed radiation is a pulsed laser.
The particles from the particle source are of a hardness which is at least close to that of the material in the region of processing and is preferably of a greater hardness. For preferred embodiments, the source of pulsed radiation is a pulsed laser.
A mechanism may also be provided for facilitating the removal of particles from an area between the tip and the area of processing, generally between radiation pulses. This mechanism may include a mechanism for vibrating the tip, the vibrations being preferably synchronized with the pulsed radiation to enhance particle delivery to the region of processing and/or the removal of particles. The mechanism for facilitating removal may alternatively include a mechanism for applying to the area between the tip and the area of processing a liquid, a gas, and/or underpressure to facilitate the removal of the particles. For certain embodiments, such delivery mechanism operates at least primarily between pulses from the source of pulsed radiation. Liquid and/or gas applied between pulses may also function to cool the surface of the area of processing. For another embodiment, air is applied before a radiation pulse to clean at least the area of processing, followed by a fine water spray or mist for at least cooling the region of processing, the radiation pulse occurring during the misting. The radiation pulse preferably lags the misting by at least a sufficient time for a thin water coating to form on the area of processing. The misting or a stronger water spray is applied after the radiation pulse.
The tip, instead of being solid, may be either hollow or liquid filled. A hollow tip may be shaped to minimize entry of particles from the particle source therein. The tip may also have an in facet cut at an angle to facilitate side processing of the material.
For preferred embodiments, the radiation is at a wavelength preferentially absorbed by the solid material. The radiation may also have a pulse duration which is of the same order or shorter than the thermal relaxation time of an absorbing fraction of the solid material. The distances between the end of the tip and a surface of the material to be processed is preferably not more than a distance of flight of the particles during which their speed decreases by a factor of 10.
The tip may be a dielectric waveguide with an end facet which is one of flat, elliptical and spherical. The tip may also include a microlens or may include some other portion focusing the radiation at or below the surface of the region of processing. A reflector may also be provided which surrounds the end of the tip and is shaped to direct the particles to the region of processing and to control the dimensions of such region.
As indicated earlier, for preferred embodiments, the solid material is a dental material such as dental enamel, dentine, bone, other dental tissue, filling material, cementum or stain. For such embodiments, the source of pulsed radiation is at a wavelength preferentially absorbed by such dental material. In particular, the source of pulsed radiation for such embodiments is preferably a pulsed laser. Examples of suitable pulsed lasers include Er:YAG with a wavelength of 2.94 xcexcm, Er:YLF with a wavelength in the 2.71-2.87 xcexcm range, Er:YGG with a wavelength of 2.7-2.8 xcexcm, CTE:YAG with a wavelength in the 2.65-2.7 xcexcm range, Ho:KGd(WO4)2 with a wavelength of 2.93 xcexcm, and CO2 with a wavelength in the 9-11 xcexcm range.
The foregoing other objects, features and advantages will be apparent from the following more particular description of preferred embodiments as illustrated in the accompanying drawings, the same reference numeral being used for common elements in the various figures.